Considering the kind of prestige class this it, you should have as a capstone that all other prestige class that raise your caster level or manifester level raise it for your 5 classes so that you dont fall behind even more after level 15.

Or you cuold also make this a 15 level prestige class (you will still have the same problem after this for epic level)

My suggestion would be to have this Prestige class be 15 level AND the capstone to continue to progress all your 5 classes at the same time when you become epic.

But beside that this seem nice